/Engine/Source/Runtime/Engine/Classes/Kismet/BlueprintSetLibrary.h
Assigns Result to the relative difference of two sets, A and B. That is, Result will contain all elements that are in Set A but are not found in Set B. Note that the difference between two sets is not commutative. The Set whose elements you wish to preserve should be the first (top) parameter. Also called the relative complement.
static void Set_Difference ( const [TSet](API\Runtime\Core\Containers\TSet)< int32 > & A, const [TSet](API\Runtime\Core\Containers\TSet)< int32 > & B, [TSet](API\Runtime\Core\Containers\TSet)< int32 > & Result )